Nanostructure guides bees to flowers

The groups of NCCR Bio-Inspired Materials Principal Investigator Professor Ullrich Steiner at the Adolphe Merkle Institute and colleagues at the University of Cambridge and London’s Kew Royal Botanical Gardens have demonstrated that many types of flowers produce a so-called “halo” – a blue shine that allows bees to identify them more easily. This color is produced by the nanostructure of a flower’s petals, which scatters light in the blue to ultraviolet range of the electromagnetic spectrum.

Electron microscopy imaging shows that on a flower petal’s surface, tiny ridges and grooves line up next to each other like a packet of dry spaghetti.  When analyzing petals from different species, the researchers discovered that the structures vary greatly in height, width and spacing. In fact, they learned that even on a single petal, these light-manipulating structures are surprisingly irregular, a phenomenon physicists describe as “disorder,” suggesting that different flowers should have different optical properties. Despite this, the flowers all produce a similar visual effect in the blue-to-ultraviolet wavelength region of the spectrum – the “blue halo” effect. The scientists concluded that these messily ordered petal nanostructures likely evolved independently many times across flowering plants, but all reached the same luminous outcome that increases visibility to pollinators – an example of what is known as “convergent evolution.”

All flowering plants belong to the angiosperm lineage. Researchers analyzed some of the earliest diverging plants from this group and found no halo-producing petal ridges. They did, however, find several examples of halo-producing petals among the two major flower groups that emerged during the Cretaceous period over 100 million years ago: monocots and eudicots. The appearance of these coincided with the early evolution of flower-visiting insects, in particular nectar-sucking bees, suggesting that this optical signal for pollinators evolved many times across different flower lineages. Species that the team found to have halo-producing petals included Oenothera stricta (a type of Evening Primrose), Ursinia speciosa (a member of the daisy family) and Hibiscus trionum (known as the “Flower-of-the-hour”). All of the analyzed flowers revealed significant levels of apparent disorder in the dimensions and spacing of their petal nanostructures.

While previous studies have shown that many bee species have an innate preference for colors in the violet-blue range, plants do not always have the means to produce blue pigments.

“Many flowers lack the genetic and biochemical capability to manipulate pigment chemistry in the blue to ultraviolet spectrum,” says Steiner. “The presence of these disordered photonic structures on their petals provides an alternative way to produce signals that attract insects.”  

By manufacturing artificial surfaces that replicated the phenomenon, the scientists were able to test its effect on pollinators, in this case foraging bumblebees. Their findings, published in the journal Nature, demonstrate that bees are able to see the blue halo and use it as a signal to locate flowers more efficiently.

The researchers recreated “blue halo” nanostructures and used them as surfaces for artificial flowers. In a “flight arena,” they tested how bumblebees responded to surfaces with and without halos. Their experiments showed that bees can perceive this difference, finding the surfaces with halos more quickly – even when both types of surfaces were colored with the same black or yellow pigment. Using rewarding sugar solution in one type of artificial flower, and bitter quinine solution in the other, the scientists also found that bees were able to use the blue halo to learn which type of surface held a reward.

The researchers say that these findings open up new opportunities for the development of surfaces that are highly visible to pollinators, as well as exploring just how living plants control the levels of disorder on their petal surfaces.

Reference: Moyroud, E.; Wenzel, T.; Middleton, R.; Rudall, P.J.; Banks, H.; Reed, A.; Meller, G.; Killoran, P.; Westwood, M.M.; Steiner, U.; Vignolini, S.; Glover, B.J. Disorder in convergent floral nanostructures enhances signalling to bees, Nature, 2017, 550, 469.